In a computer-based system, such as a desktop PC for example, peripheral component interconnect (PCI) devices are often part of the system. PCI devices are peripheral devices such as an Ethernet controller, audio controller, IEEE 1394 controller, cardbus controller, and SCSI controller.
A CPU is the computing and control hardware element of a computer-based system. In a personal computer, for example, the CPU is usually an integrated part of a single, extremely powerful microprocessor. An operating system is the software responsible for allocating system resources including memory, processor time, disk space, and peripheral devices such as printers, modems, and monitors. All applications use the operating system to gain access to the resources as necessary. The operating system is the first program loaded into the computer as it boots up, and it remains in memory throughout the computing session.
Some PCI devices may be on-board devices (devices on the motherboard of the computer-based system). For example, many computer-based systems have a SCSI or Ethernet controller on the motherboard. Other PCI devices are add-in devices that are plugged into PCI card slots within the computer-based system.
Most logic devices today use synchronous or clocked implementations. Power management schemes are often used to minimize the number of unnecessary state transitions since any change of state in a semiconductor device consumes power.
One power management scheme called clock control may involve slowing the device clock or stopping the clock altogether. In a PCI environment, the clock run (CLKRUN#) protocol (i.e. CLKRUN# support capability) may be used to achieve clock control. A central resource (e.g. a Southbridge) in the computer-based system is typically used as the clock controller to control the modulation of a PCI clock and to monitor and drive the CLKRUN# line to implement the PCI clock run protocol.
An example of power control of a PCI device is the control of a graphics controller. A graphics controller consumes significant power and, therefore, turning the graphics controller off saves substantial power and also puts the monitor in a power savings mode. In certain systems, the diskette motor is controlled by a BIOS (basic input/output system). In a PC, a BIOS is a set of instructions that configures the hardware and starts the loading process of the operating system when the PC is first powered up.
The BIOS may also perform PCI enumeration at system boot time. The BIOS instructs a PCI bus driver to scan the PCI bus for PCI devices. When a PCI device is found on the PCI bus, a registry is searched for a corresponding instance key or template. A best match is searched for using device identifiers such as “class”, “subclass”, “vendor ID”, etc. If enabled, the PCI device is configured for use during run time. Otherwise, the device may be disabled and powered down.
In a PCI environment, CLKRUN# is an optional signal that may be used to request starting, stopping, slowing down, or speeding up the PCI clock for PCI devices within the system. CLKRUN# also indicates the status of the PCI clock. For PCI devices, CLKRUN# is an open drain output and also an input. For the central resource (provider of the PCI clock), CLKRUN# is a sustained tri-state I/O signal. The central resource is responsible for maintaining CLKRUN# in the asserted state and for driving CLKRUN# to the de-asserted state.
There are three main states in the PCI clock run protocol. They are:                Clock running—the PCI clock is running and the PCI bus is operational.        About to stop/slow down—the central resource has indicated on the CLKRUN# line that the PCI clock is about to stop or slow down.        Clock stopped/slowed—the PCI clocked is stopped or slowed, with CLKRUN# being monitored for a restart.        
The central resource drives CLKRUN# low while the PCI clock is running at a normal operating frequency. Before slowing down the PCI clock to a non-operational frequency or stopping the PCI clock, the central resource drives CLKRUN# high and tri-states its driver. PCI devices are required to maintain their states while the PCI clock is stopped or the clock rate is changed.
A PCI device may assert the CLKRUN# signal to request the central resource to restore the PCI clock to a normal operating frequency. After detecting the assertion of CLKRUN#, the central resource starts the PCI clock if the PCI clock was stopped, or brings the PCI clock to a normal operating frequency if the PCI clock was just slowed.
When the PCI clock is slowed or stopped by use of CLKRUN#, power savings are achieved. According to the PCI clock run protocol, all CLKRUN# lines of all PCI devices are tied together. Therefore, all CLKRUN# compatible PCI devices will be powered down at the same time. All PCI devices must acknowledge to the central resource that they are ready to be powered down before the central resource may power down the PCI devices.
It has not been a requirement, however, that PCI devices be compatible with the CLKRUN# protocol. Particularly, add-in PCI cards to PCI slots are not compatible with the CLKRUN# protocol. Certain on-board PCI devices such as a SCSI controller, for example, may not be compatible with the CLKRUN# protocol.
As a result, if a PCI environment of a computer-based system is such that there are some CLKRUN# compatible PCI devices and some non-CLKRUN# compatible PCI devices on the PCI bus that all require use of the PCI clock, then the CLKRUN# protocol may not be used to slow or stop the PCI clock unless the non-CLKRUN# PCI devices are disabled. Therefore, in many desk-top systems, for example, that have PCI card slots, PCI device power management is not performed. It is not centrally known by the central resource which PCI devices are present and which PCI devices are enabled or active, and the CLKRUN# protocol is not linked to PCI device state to provide PCI power management as a function of the state of the PCI environment.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings.